The present invention relates to a process for preparing a polycrystalline silicon thin film, more specifically, to a process for preparing a polycrystalline thin film comprising a step of microwave annealing and crystallization of an amorphous thin film of silicon semiconductor, silicon semiconductor added with impurities, IV family semiconductor comprising Si alloy such as Si1xe2x88x92xGex, III-V family and II-VI family semiconductor.
In recent years, a variety of processes for obtaining polycrystalline thin film on amorphous substrate have been suggested in the art, to meet the requirement of polycrystalline thin films to be applied to polycrystalline silicon(poly-Si) thin film transistor(xe2x80x9cTFTxe2x80x9d) for SRAM, TFT for LCD, solar cell, SOI device, etc. Particularly, attempts to obtain polycrystalline thin film under a low temperature, which is critical factor for the consideration of solar cell or TFT for LCD employing glass as a substrate, have been tried as followings:
Japanese patent laid-open publication (Sho) 4-144122 and (Sho) 3-25633 disclose a process for fabricating polycrystalline thin film by depositing amorphous thin film on a substrate and then crystallizing by way of annealing at a relatively high temperature of about 600xc2x0 C., in accordance with low-pressure chemical vapor deposition(LPCVD) method.
Inverson et al. also teaches a process for fabricating crystalline thin film with increased grain size by depositing polycrystalline thin film on a substrate, implanting impurities to have amorphous characteristics, and finally annealing at a temperature of about 600xc2x0 C.(see: R. B. Inverson et al., J. Appl. Phys., 65:1675(1987)).
Furthermore, Nakazawa et al. reports that polycrystalline thin film may be fabricated by depositing amorphous thin film on a substrate employing Si2H6 and then, annealing at a temperature of about 600xc2x0 C., in accordance with plasma enhanced chemical vapor deposition(PECVD) method(see: Nakazawa et al., J. Appl. Phys., 68:1029(1990)).
The prior art methods are, however, proven less satisfactory in the senses that: low-priced glass cannot be employed as a substrate, since a high temperature of above 600xc2x0 C. is essentially required to crystallize amorphous thin film; a step of implantation of impurities is also accompanied; and, expensive gas such as Si2H6 is needed, all of which increase the costs for manufacturing process.
As an approach to overcome the problems in the art, an annealing method which employs laser to crystallize through melting, after deposition of thin layer is made in accordance with PECVD or LPCVD method, has been provided. However, this method has also revealed shortcomings that: high cost is required for facilities of manufacturing and films of uniform thickness can be hardly achieved.
On the other hand, a process for obtaining polycrystalline thin film by depositing metals such as copper, gold and platinum as impurities to thin film, has been suggested in the art(see: D. K. Sohn et al., Jpn. J. Appl. Phys., 35:128(1996); S. W. Lee et al., Appl. Phys. Lett., 66:1571(1995)). Although this method permits the use of glass substrate due to the lowered temperature for crystallization, it has also revealed a problem that metals used for crystallization remain inside of polycrystalline thin film as impurities which in turn deteriorates the characteristics of device.
Accordingly, there are strong reasons for exploring and developing an alternative means for preparing a polycrystalline silicon thin film in an economical and simple manner.
In this regard, the present inventors have made an effort to develop an improved method for fabricating a polycrystalline silicon thin film, and provided a novel process which successfully tides over the said problems by employing a step of microwave annealing for crystallization. In accordance with the process of the invent-ion, efficient and economical fabrication of homogenous polycrystalline thin film can be realized, since it employs microwave annealing of amorphous thin film by which crystallization temperature can be lowered to a degree that allows the use of glass as a substrate.
A primary object of the present invention is, therefore, to provide a process for preparing a polycrystalline silicon thin film employing microwave, which allows crystallization at a low temperature.